Post contributed by Dr. E. V. Bohacek, European Space Agency (ESA)
In contrast to Earth, where water is cycled through rivers, oceans and the atmosphere, Saturn’s largest moon Titan is a methane world. Liquid methane falls as rain on Titan’s surface, which forms fluvial landforms (rivers), lakes and seas. It is believed that these methane rivers experience episodic activity, with rainfall primarily occurring at the poles and persisting for 10-100 hours per Titan year, equivalent to 30 Earth years (Lorenz, 2014). Another big difference between the fluvial landforms of Titan and the rivers of Earth is their drainage patterns: the distinctive arrangement and organisation of rivers and streams within a river basin. Most rivers on Earth have a dendritic drainage pattern, resembling a branching tree (Image 1 – Yemeni River), whereas on Titan the majority of fluvial landforms branch at right angles in rectangular drainage patterns [Burr et al., 2013] (Image 1 – Titan River). Titan is also covered by vast regions of linear dune fields (Image 1 – Titan Dunes), similar to those observed in the Namib Desert on Earth (Image 1 – Namib Dunes), primarily within the equatorial latitudes [Rodriguez et al., 2014], thought to be composed of hydrocarbon and nitrile sand-sized particles formed from photochemical reactions in Titan’s atmosphere (Hirai et al., 2023). Although observations of Titan are limited to those taken by Cassini-Huygens from 2005-2017, interactions between rivers and dunes have been observed. It is possible that these interactions could partially explain the presence of rectangular drainage patterns.
Image 1: (Top Left) An example of a typical river valley system on Earth with a dendritic drainage pattern in Yemen [Source: Landsat]. (Top Right) Fluvial landform annotated in blue by Burr et al., (2013) in radar imagery of Titan, showing a rectangular drainage pattern [Source: NASA Cassini]. (Bottom Left) Radar image of linear dunes in the Namib Desert on Earth. The linear dunes, similar in size to those on Titan, are 1-2 km apart. (Bottom Right) Linear dunes in Titan’s sand sea imaged by Synthetic Aperture Radar [Source: NASA Cassini].
Dunes and rivers are likely active and interacting on Titan today, however there is no way to observe them. To understand these interactions prior to upcoming Titan missions (NASA’s Dragonfly), we can utilise a landscape evolution model to simulate Titan’s river-dune dynamics, leveraging our current knowledge of Titan’s surface conditions. This post introduces the Working Group on Aeolian-Fluvial Terrain Interactions (WAFTI) at the European Space Agency’s initial study aiming to explore these processes’ combined effects under Titan’s conditions through modeling and geomorphological analysis.
We take an elevation map of linear dunes from the Namib Desert that are similar to linear dunes on Titan and start a river on the left edge of the simulation. There is a gentle slope from the top and bottom edges towards the center and a stronger slope from the left to the right edges – this means that if the dunes were not there, the river would flow left to right down the center of the domain. In the example in image 2 we see that upon encountering the dunefield, the river’s flow is obstructed by dune topography, leading to deposition and northward migration. An initial attempt to penetrate into the dune field via an interdune corridor is blocked by a collapsed dune, resulting in pooling and headward erosion. Consequently, the river abandons this course, migrating northward until it is redirected down another interdune corridor, being funneled against the local slope direction. As a result, the river transports material deeper into the dune field and forms a channel with a pronounced right-angled planform, similar to rectangular drainage patterns.
Image 2: A simulated flood event lasting 1 Titan year (30 Earth years) in a static dune field
In this next example, Image 3, we see that when the dune crests are perpendicular to the river, the dune topography prevents the river from penetrating into the dune field, leading to ponding. The reduction in fluvial velocity leads to deposition of sediment, resulting in an alluvial fan or deltaic structure. Our models suggest that when Titan rivers and dunes interact, this could strongly affect the drainage patterns, and also explain an additional mechanism for the addition and redistribution of sediment through Titan dunefields.
Image 3: A simulated flood event lasting 1 Titan year (30 Earth years) in a static dune field, with river incursion perpendicular to dune crests.
Further Reading:
Al-Masrahy, M. A., & Mountney, N. P. (2015), A classification scheme for fluvial–aeolian system interaction in desert-margin settings, Aeolian Research, 17, 67-88.
Burr, D. M., et al. (2013), Morphology of fluvial networks on Titan: Evidence for structural control, Icarus, 226, 742-759.
Hirai, E., et al. (2023), Rapid Aggregation and Dissolution of Organic Aerosols in Liquid Methane on Titan, Geophysical Research Letters, 50, e2023GL103015.
Liu, B., & Coulthard, T. J. (2015), Mapping the interactions between rivers and sand dunes: Implications for fluvial and aeolian geomorphology, Geomorphology, 231, 246–257.
Liu, B., & Coulthard, T. J. (2017), Modelling the interaction of aeolian and fluvial processes with a combined cellular model of sand dunes and river systems, Computers and Geosciences, 106, 1–9.
Lorenz, R. (2014), Physics of saltation and sand transport on Titan: A brief review, Icarus, 230, 162-167.
Rodriguez, S., et al. (2014), Global mapping and characterization of Titan’s dune fields with Cassini: Correlation between RADAR and VIMS observations, Icarus, 230, 168-179.
Planetary Geomorphology Image of the Month 